1. Technical Field of the Invention
The present invention relates to the mobile communications field and, in particular, to a method and system for determining time slot offsets in a non-synchronized Time Division Multiple Access (TDMA) network.
2. Description of Related Art
TDMA mobile communications systems can be either inter-cell synchronous or inter-cell asynchronous systems. In other words, the base stations (BSs) in an inter-cell synchronous system are accurately synchronized with one another, and the BSs in an inter-cell asynchronous system are not. More specifically, asynchronous BSs do not share a common time reference, and their transmissions, therefore, have arbitrary timing relative to each other. An example of an inter-cell synchronous system is the North American IS-95 system. Examples of inter-cell asynchronous systems include the Wideband Code Division Multiple Access (WCDMA) systems proposed in the CODIT, ETSI SMG2 Group Alpha, and ARIB technical specifications and Ericsson""s GSM system protocols.
A number of disadvantages exist with inter-cell synchronous systems. One prerequisite for such systems is a high level of synchronization among the BSs with the degree of synchronization generally measured in microseconds (xcexcs). Also, it is believed that a loss in frequency reuse efficiency is realized in an inter-cell synchronous network, at a cost of valuable bandwidth.
However, an area in which the synchronous network has shown particular advantage over non-synchronous networks is in mobile positioning applications. Synchronous networks have a distinct advantage over non-synchronous networks since, by design, the synchronous networks share a reference clock. Triangulation algorithms may then be utilized to measure delays in the time of arrival of specific time slot numbers from a particular mobile station (MS) to a base station. With, in general, measurements from a MS to three BSs, the position of an MS within a telecommunications system may be accurately determined.
When BSs are operating asynchronously, however, the task of location calculation is complicated by the fact that each BS is operating on clocks independent from one another. For example, while one BS is receiving data on Time slot Number 1 (TN1), another BS might be simultaneously receiving on TN3. To make an effective location determination, the respective delay of the MS transmission to the BS reception must be ascertained along with the relative TN offset with respect to the other BSs participating in the location calculations.
A better understanding of the problems associated with positioning in asynchronous networks may be had with reference to FIGS. 1 and 2. FIG. 1 illustrates a typical framing scheme as employed in a digital TDMA communications system. The numbering scheme depicted is specific to the European GSM system and is used only for illustrative purposes. Shown are contiguous frames, denoted F1 through F2715647, the span of which is often referred to as a hyperframe, and one single frame of the hyperframe, specifically frame 2 (F2), is shown in more detail. In GSM, the transmission of such frames occurs on carrier frequencies with an approximate bandwidth of 200 kHz separating adjacent transmissions.
As illustrated by the expanded frame F2, each single frame contains eight individual time slots numbered TN0 through TN7 therein. One time slot of a TDMA frame on one carrier is referred to as a physical channel. Therefore, a mobile station transmitting on a particular carrier will occupy a specific time slot, or physical channel, on contiguous frames thereby allowing up to eight such xe2x80x9csimultaneousxe2x80x9d communications on the carrier frequency depicted on the respective time slots TN0 to TN7.
Mobile stations are, in general, within reception range of a number of BSs with a traffic channel maintained between the mobile station and that BS exhibiting the best communication characteristics, i.e., signal to interference ratio. As is understood in the art, however, when RF characteristics decline below a specified level or when RF characteristics from another BS increase beyond a specific threshold relative to the current BS with which the mobile station is maintaining the traffic channel, a handover is initiated where a traffic channel is setup between the mobile station and the BS exhibiting the better communication characteristics and, concurrently, the traffic channel between the mobile station and BS previously in use is broken. However, in specific situations, i.e., MS positioning, a number of BSs may concurrently tune to the same transmitting MS in order to make time delay of arrival measurements between the MS and the BSs.
FIG. 2 illustrates a network containing three BSs 50, 60, and 70, in a telecommunications network, generally designated by the reference numeral 90, operating asynchronously and sharing a common node, or Base Station Controller (BSC) 80. Each of the BSs are transmitting data in a TDMA format consistent with the aforedescribed data frames such as shown in FIG. 1. Since the BSs are operating asynchronously, however, their frame transmission times will have no time correspondence other than that by coincidence.
For convenience of discussion, BS 50 is taken as a reference in FIG. 2 and the beginning of its current frame transmission is designated as occurring at a time of zero seconds (t11=0). Here, the first digit of the double subscript indicates both the number of the BS, e.g., BS 50 or the first (xe2x80x9c1xe2x80x9d) BS, while the second subscript refers to the reference base station from which corresponding times are measured, i.e., BS 50, again the first (xe2x80x9c1xe2x80x9d) BS. As is understood in the art, the time span of a single GSM frame transmission is approximately 4.615 ms. With reference now to the frame transmission corresponding to BS 60, which as discussed is asynchronous to the other BSs and offset from the other frame transmissions such as that of BS 50, BS 60 (the second xe2x80x9c2xe2x80x9d BS) begins its current frame transmission, F5000, at t21=1.026 ms, or 1.026 ms after BS 50 began transmitting its frame F3, the base reference time in this example. As discussed, BS 60 completes the transmission of frame F5000 about 4.615 ms after commencing transmission, i.e., at time t21=5.641 ms, at which point frame F5001 commences. Likewise, BS 70 (the third xe2x80x9c3xe2x80x9d BS) begins transmission of its current frame F11358 at time t31=3.969 ms after BS 50 began transmitting its frame F3 and 2.666 ms after BS 60 began transmitting its frame F5000. Completion of transmission of frame F11358 by BS 70 arrives one frame time length later, i.e., t31=8.307 ms, as illustrated in FIG. 2.
Since the frame lengths and time slot lengths are of constant durations, a single time slot has a span of 0.577 ms. Lines 100 and 110 may then be constructed to gain further insight into the time slot offsets between particular BSs within the telecommunication network 90. Line 100, drawn from the start point of frame F5000 ofBS 60 to intersect the corresponding time point within frame F3 of the reference BS 50, indicates that BS 60 began transmitting its time slot zero (TN0) within frame F5000 at a point in time where BS 50 has completed transmission of its TN0 and additionally about 78% of its TN1. Similarly, line 110, drawn from the start point of frame F11358 to intersect the corresponding time points within frame F5000 of BS 60 and frame F3 of the reference BS 50, indicates that BS 70 began transmitting its TN0 at a point in time where BS 60 is in progress of transmitting its TN4 and, where BS 50 is in progress of transmitting its TN6.
The transmission offsets of the aforedescribed network are simply the result of non-synchronous operation. The BSs are free to begin transmissions when needed and without correspondence between ongoing transmission in nearby BSs. Thus, any BS frame transmission in an asynchronous network may begin at a time corresponding to the transmission of the beginning, end, or any fraction thereof of a frame transmitted by a nearby BS. Clearly, it would be advantageous for transmitting BSs to be able to measure their frame transmissions times with reference to a common clock so that any transmission offsets therebetween could easily be determined.
It is, accordingly, a first object of the present invention to provide an improved system and method for measuring asynchronous base station time slot offsets in a mobile telecommunications network.
It is also an object of the invention that the system and method of the present invention substantially adhere to the TDMA protocols.
It is a further object of the invention to provide a time slot offset measurement system and method where a controlling node is capable of collecting time slot offset data on a number of base stations where the time slot offset data may be collected intermittently and independently from a particular reference base station with respect to any of the other base stations controlled by the controlling node.
In accordance with the present invention, a system and method are provided for measuring the time slot offsets in an asynchronous digital TDMA network of transmissions originating from different transmission sources driven by individual non-common internal clock signals. The transmission sources are equipped to receive a common reference clock signal, whereupon the transmission sources"" individual non-common internal clocks are each phase locked with the common reference clock signal. A number of transmission sources are coupled to a common node by signaling channels that may occupy either a hard-wire or radio frequency (RF) link. Each individual transmission source is capable of reading the value of the reference clock signal value corresponding to a particular moment a specific time slot is transmitted. The value of the reference clock signal corresponding to a particular time slot transmission is then recorded by the transmission source. The transmission source then performs a modulo operation on the recorded time and reports the result to a controlling node. Results of the modulo operation may then be compared with similar results reported by other transmission sources to the controlling node to determine relative time slot offsets between the transmission sources. In another embodiment of the present invention, the transmission sources may forward the time as measured by the reference clock corresponding to a particular time slot transmission directly to the controlling node whereupon the controlling node performs the modulo calculations. In still another embodiment of the present invention, the transmission time is measured by a monitoring node next to the transmission source.